Obviously jealousy is not limited to just men.
Jealousy is a tricky thing. We see or hear about something that causes a horrible emotional reaction inside of us. Generally that emotion stems from fear, anger, and insecurity and it's often overwhelming and difficult to control.
in my opinion, there are two kinds of jealousy: rational and irrational jealousy.
I think rational jealousy is okay. It tells us that there's actually something wrong occurring---your partner is actually neglecting your needs or your relationship. If your jealousy is for rational reasons, then its something you need to talk about and work out with your partner.
Irrational jealousy is when you get these feelings, but they aren't really warranted. Like your partner is talking to another person, or another person hits on him/her but he/she politely declines their advances. If none of your partner's actions suggest that he/she's going to cheat or is neglecting your relationship, then it's usually just you projecting your own feelings of insecurity on your partner.
People have some weird ideas about jealousy. People think that its okay to be jealous and that its expected, and if someone isn't jealous, it means that they don't care about their partner. In reality, lack of jealousy generally means that you TRUST your partner. In my opinion, jealousy is a sign of insecurity and mistrust, and NOT a sign of love.
I think that the difference in jealousy between men and women is that men have the additional pressure to be dominant, territorial, and to "be a man". They see another man talking to their girl as undermining their manliness, and perhaps, since they "know how guys think" assume that he has shady intentions concerning his girl. This is probably compounded by comments that his guy friends may make if they see his girl talking to another guy. He doesn't want his friends to think that he's getting "played" or that his girl has "wandering eyes" because he doesn't satisfy her.
Girls experience this too, however. Many feel the need to be territorial of their man, feel mistrustful of other girls, and don't want other people to think that she doesn't satisfy him; also, I think girls feel the need to protect their guy's reputation amongst their friends---they don't want their girl friends to see him talking to another girl and assume he's an asshole who doesn't care about her.